The present invention relates to permanent magnet generators and, in particular, to generators of this type intended for use as auxiliary machines in association with larger main generators and in close physical proximity thereto.
Permanent magnet generators of relatively small size are often used as auxiliary machines to supply excitation or control power, or for other purposes, in association with a larger main generator. Such machines are frequently used, for example, in connection with aircraft generators. Since the size and weight of aircraft generators must be kept as small as possible, the auxiliary permanent magnet generator is built into a common housing with the main generator so that it is closely physically associated therewith. It has been found that severe modulation, or fluctuation, of the output voltage of such auxiliary machines frequently occurs at the frequency of the main machine and may be as great as 40% 50% of the nominal voltage. It has also been found that the permanent magnet generator can be partially demagnetized when a short-circuit occurs on the output terminals of the main generator.
These effects are, of course, highly undesirable but they are due to the necessarily close physical proximity of the permanent magnet generator to the field winding and the alternating current armature winding of the main generator. This physical relation causes the auxiliary permanent magnet generator to be within the range of stray magnetic fields of the main generator windings which are strong enough to cause both fluctuations in the output voltage of the auxiliary generator and partial demagnetization of the permanent magnets in its magnetic circuit. The dimensional limitations which these machines must meet, to be acceptable for aircraft use, are such that this close physical relation of the auxiliary generator and the main generator is necessary and the auxiliary generator cannot be spaced far enough away from the main machine to avoid the stray magnetic fields. It has been proposed to overcome this problem by means of copper shields placed between the auxiliary and main generators, but such shields do not greatly reduce the modulation of the auxiliary generator output voltage and add undesirable weight to the complete assembly.